Two Precisely bored and contour milled holes, produced at 0 deg and
180 deg of the indexer, must look at each other within less than
0.0005".

For machining precision components, probing can yield faster setup
on almost any part and, when combined with automatic tool setting and
self-regulating machine tools, will yield higher precision and
efficiency.

That's the experience at Toledo Scale's Spartanburg, SC,
plant where the combination is increasing accuracy and output of milled
and bored "counterforces," which are critical mechanical parts
used on its electronic weighing systems. The basic counterforce can be
described as a beam, having an extremely precise cross section to which
electronic strain gages are attached.

Major differences in weighing capacity and the environment in which
the counterforce must work result in hundreds of different
configurations. To avoid corrosion and permanent set from overloads, the
parts are usually made from hard-to-machine materials such as stainless
steel and pre-hardened steel alloys.

The demand for ever-increasing accuracy in these precision parts
led Jeff Pickens, manufacturing manager, Tom Johnson, senior
manufacturing engineer, and Steve Jennings, manufacturing engineering supervisor, to examine new approaches to the most common problems in
machining the counterforces. The problems included:

* machine positioning accuracy and alignment,

* tool wear and variation in tool size with change of inserts,

* temperature drift of the part and of the machine,

* registration errors in the fixture,

* distortions during clamping,

* variations in material hardness,

* human errors in tool setting,

* in-process inspection errors which lead to incorrect adjustments.

Though each error might be small in itself, their combined
influence can cause excessive scrap, rework, and reduced output. The
solution decided upon was combining all machining operations on the
counterforce in a single setup on

a Monarch VMC-45B with a fourth-axis rotary table.

Monarch Cortland, Cortland, NY, was charged with the responsibility
to design and build fixturing and provide all processing and software
development as part of a complete turnkey package. While the machine and
fixturing were being built, an in-depth analysis of advances in
perishable tooling was made to benefit from any major improvements in
cutting tools in terms of cutter life and replacement accuracy.

A major departure from conventional practice was the decision to
use probing for automating the setup process and as the method for
in-process inspection.

A three-pin driving fixture was proposed that would prevent
distortion of the workpiece due to clamping. Programming and processing
were to be arranged so that all setup activities including fixtures
registration, tool length offsets, and location of part centerline would
be done automatically by a Renishaw spindle probe and a Monarch Tool
Length Indicator (TLI) to eliminate human error.

In-process inspection by the probe would monitor tool wear and
automatically enter revised offsets, if required.

The effectiveness of probes as setup and inspection tools is
frequently debated; detractors describe probes as delicate, costly, and
time-consuming to use. When combined with an accurate CNC machine and
driven by proper software, however, probes are capable of results that
cannot be achieved otherwise. Here's how the probe is used in a
setup procedure:

As with any measuring instrument, the probe must be calibrated regularly. X and Y calibration is done by measuring a master bore
diameter of known size. Numerical adjustments, known as ball center
offsets, are included in the CNC subroutines which control the probe and
are automatically modified during calibration to account for the
diameter of the probe tip, any runout of the probe tip relative to the
spindle centerline, and any non-linearity of the probe itself.

Z calibration of the probe establishes the length of the probe
relative to the tool length indicator by touching the probe and a master
tool to the same reference Z surface. The master tool is then measured
by the tool length indicator that defines the Z dimension between the
probe and the tool length indicator. All of the measurements established
during calibration are stored in the CNC and can be called up as
required for inclusion in various setup or measurement subroutines.

Setup of a typical counterforce begins with tool measurement. A
pre-programmed cycle automatically loads the tool in the spindle which
traverses in Z to the TLI.

Each tool length is automatically measured and stored in the CNC in
about 15 seconds. The 14 tools required are set in less than four
minutes. The spindle-mounted probe requires five touches of the part
blank to locate the part in X, Y, and Z, as well as finding the center
of rotation of the indexer in Y and Z.

Probe cycles for finding centerlines are similar to manual
procedures using dial indicators. Figure A illustrates the method of
finding the indexer centerline.

Following completion of the above steps, all information necessary
to commit the tools to cut has been stored in the CNC as follows:

1) The length of all tooling is known relative to the probe.

2) The probe location in Z relative to the workpiece surface is
known.

3) The distance from the workpiece surface to the centerline of
rotation of the part indexer is known, thus defining the Z location of
the centerline.

4) The part program defines the Z motions of the tool in relation
to the centerline of rotation.

5) The centerline of rotation is known in Y.

Tool length offsets for Z and workpiece offsets for X and Y are
automatically set in the CNC and cutting begins. The dimensional outcome
of the part will be dependent upon automatic measurements and
independent of operator intervention. Often some "attitude
adjustment" is required by new operators to avoid panic stops as
they gain confidence in the hands-off procedures of automatic setup.

Figure B shows a cross section of a typical counterforce. The four
"A" dimensions must be held to the same size within + or -
0.0005". The surfaces associated with dimension B must be flat
within 0.0005" and dimension B must be the required dimension
within + or - O.001". Finishes required are 32 microinches.

Stated in a different way, two precisely bored and contour milled
holes, produced at 0 deg and 180 deg of the indexer, must "look at
each other" within less than 0.0005". The end mill, which
finishes surface B, must hold 0.0005" tolerance in Z.

After the sides of the part have been milled to finish size, the
probe is reinserted in the spindle. The sides are measured for correct
width and centrality with the indexer prior to beginning the opposing
bores.

Four probe touches are required for this in-process inspection. If
required, offsets in Y are automatically fine-tuned and the part is
completed. Following completion of the part, machine performance is
confirmed on a coordinate measuring machine.

Runoffs during the recent machine installation training at Toledo
Scale resulted in two new operators running off five different parts in
lot size of ten. Fifty good parts were produced using full automatic
setup and in-process gaging.

For information, circle 192.

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